This invention relates to a control valve for use in hydraulic apparatus which operates as a combined relief and make-up valve in response to a high or low pressure in an accumulator of the hydraulic apparatus.
Generally, in hydraulic apparatus a control valve is used as a combined relief and make-up valve so as to operate in response to a hydraulic pressure supplied by hydraulic pump means. As the control valve of this kind there is usually used a direct operating type control valve in which pressurized oil is supplied through one opening formed through a valve housing into the valve and connected to an oil tank, and the diameter of a valve seat contacting with a relief poppet of the control valve is designed to be considerably large so as to reduce resistance against the pressure of the oil passing through a passage of the relief valve. For this reason, a considerably large or strong spring means must be used to act against the oil pressure to suitably set the oil pressure in the control valve.
Therefore, such a direct operating type control valve cannot be made to be compact because it is inevitable to use a considerably large and strong spring means and, otherwise, the reliability of the spring would not be assured.